The present invention generally relates to flowmeters and more particularly to a cross correlation flowmeter for measuring a flow rate of a fluid flowing through a conduit by detecting a movement of a disturbance of fluid transported along the conduit by means of at least two sensors disposed on the conduit along a longitudinal direction thereof, in which the flow rate is obtained on the basis of a cross correlation of two output signals produced by the two sensors.
A fluid flowing through a conduit usually includes a disturbance such as a disturbance of flow velocity or temperature of the fluid naturally formed in the fluid to some extent. The disturbance is transported through the conduit with the fluid. The disturbance continuously changes as it is transported with the fluid. Conventionally, there is a cross correlation flowmeter for obtaining a flow rate of a fluid flowing through a conduit by detecting a movement of such a disturbance. For example, Critten et al. discloses in U.S. Pat. No. 4,019,038 a cross correlation flowmeter using two sensors respectively disposed at an upstream position and downstream position on a conduit. Further, R. S. Medlock describes a general technique of cross correlation flowmeter using two sensors in an article entitled "Cross Correlation Flow Measurement", Brown Boveri Review 6/7, 311-318, 1984. The prior art cross correlation flowmeter generally comprises a conduit through which a fluid is flowing, a first sensor disposed on the conduit at an upstream position for sensing a disturbance of the fluid transported along the conduit with the fluid, a second sensor disposed on the conduit at a downstream position relative to the first sensor with a predetermined distance therebetween for sensing the same disturbance sensed by the first sensor after a time determined by the distance between the two sensors and the flow velocity of the fluid, and means for calculating cross correlation of output signals from the first and second sensors and for obtaining a time difference between the output signals which maximizes the cross correlation of the two output signals. As the disturbance moves with the fluid, the time difference indicates a transit time required for a disturbance to move from the first sensor to the second sensor in the conduit. By dividing the distance between the two sensors by the transit time, the flow velocity is obtained and a flow rate is calculated on the basis of the flow velocity thus obtained.
In cross correlation flowmeters, measurement of the transit time generally involves an uncertainty due to a finite sharpness of a cross correlation peak indicating the maximum of the cross correlation. Accordingly, in order to achieve an accurate measurement, it is necessary to choose the distance between the upstream sensor and the downstream sensor as long as possible. However, the shape of the disturbance such as the disturbance of the flow velocity or temperature in the fluid generally changes as it is transported with the fluid through the conduit. As a result, a similarity of the disturbances observed at the upstream sensor and the downstream sensor disappears in case the distance between these two sensors is excessive. Thus, the cross correlation peak indicating the maximum of the cross correlation becomes diffused and the accuracy of the measurement becomes poor. An optimum distance of the sensors giving a maximum accuracy of the measurement changes with the type of the fluid, the fluid velocity and the like.
In order to overcome the aforementioned problem and expand the measuring range, Iwamura proposed in the Laid Open Japanese patent application No. 59-173715 a cross correlation flowmeter having an upstream sensor and a plurality of downstream sensors. In the cross correlation flowmeter by Iwamura, respective downstream sensors are associated with respective correlators each of which calculates a cross correlation of the output signals from the upstream and downstream sensors to which the correlator is cooperating, whereby an optimal combination of the upstream sensor and the downstream sensor is selected responsive to the flow velocity of the fluid and the accuracy of the flow rate measurement is maximized. However, the cross correlation flowmeter by Iwamura requires a correlator for each of the plurality of the downstream sensors and the circuit construction is complicated. Further, a large number of correlators are required. Furthermore, as each of the correlators perform the cross correlation calculation, calculation time necessary for obtaining the flow rate becomes long.
Accordingly, it is an object of the present invention to provide a novel and useful cross correlation flowmeter in which the problems aforementioned are eliminated.
Another and more specific object of the present invention is to provide a cross correlation flowmeter for measuring a flow rate of a fluid flowing through a conduit comprising a first upstream sensor disposed on the conduit for sensing a passage of a disturbance of the fluid continuously changing as it is transported with the fluid through the conduit, a second upstream sensor disposed on the conduit adjacent to the first upstream sensor for sensing the passage of the disturbance substantially identical in shape with the disturbance sensed by the first sensing means, a plurality of downstream sensors disposed on the conduit along an elongating direction thereof at a relatively downstream side to the first and second upstream sensors with predetermined distances from the first upstream sensor, first flow velocity detection means for detecting an approximate flow velocity by calculating a cross correlation of output signals from the first and second upstream sensors, selection means for selecting one-of the plurality of the downstream sensors which is optimum for the accurate flow rate measurement responsive to the first flow velocity, and second flow velocity detection and flow rate calculation means for detecting a second flow velocity by calculating a cross correlation of an output signals from the first upstream sensor and from the selected one of the downstream sensors and for calculating the flow rate of the fluid from the second flow velocity. According to the present invention, the accuracy of the flow rate measurement is maximized by selecting an optimum combination of the sensors providing an optimum distance for the measurement. Further, the circuit construction is simplified as only one correlation calculation means is required for the plurality of the downstream sensors. Furthermore, the calculation time is saved as the cross correlation calculation is not necessary for all of the upstream and downstream sensor combination. Thus, the response of the cross correlation flowmeter is improved.
Another objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.